Similar to ovine ICSH, the human hormone has two non-identical subunits. The amino acid sequence of human ICSH-a has been elucidated; it consists of 89 amino acids with the single residue in position 22, and the three methionine residues in positions 26, 44, and 68. The two carbohydrate moieties are linked to asparagine residues at positions 49 and 75. In the rat hypophysectomized on day 5 of pregnancy, replacement hormonal treatment daily with 100 Mg ICSH or 2 mg Prolactin alone was not effective for maintenance of pregnancy, whereas the combination of 50 to 200 micrograms ICSH and 2 mg prolactin gave live embryos in 40 to 90% of treated animals. In the animal hypophysectomized at more advanced stage of pregnancy, ICSH alone was able to maintain pregnancy. When 400 micrograms ICSH-beta was used instead of 100 micrograms ICSH in conjunction with 2 mg prolactin, this treatment was also effective for maintenance of pregnancy in the animal hypophysectomized on day 8, either reconstituted ICSH or ICSH-beta alone could maintain pregnancy. ICSH-alpha was ineffective in the animal hypophysectomized on either day 5 or day 8 of pregnancy. Thus, it appears that ICSH-beta is the biologically active subunit in the ICSH molecule for maintenance of pregnancy in the rat.